This invention relates to an electrostatic coating apparatus which is used in painting work for applying a liquid paint and, more particularly, to an air atomizing electrostatic coating gun which atomizes a liquid paint into very small particles by using air flows and which applies atomized particles by electrostatic force on an object to be coated.
An air atomizing electrostatic coating gun of conventional type comprises, a paint nozzle for ejecting a liquid paint there-through which is disposed at the front end of the body of the gun; a paint atomizing air cap having a central air orifice and a plurality of air orifices surrounding the central orifice which is also disposed at the front end of the gun body; a needle valve device for controlling ejection of the paint from the nozzle, which comprises an elongated rodlike valve having a front end portion generally in the shape of a cone which is brought into contact with or detached from a valve seat formed by a slanted inner wall of the paint nozzle, a valve driving piston fixed to the rear end of this valve, and an air cylinder accommodating this piston, and is incorporated in the gun body; and a thin elongated electrode for charging paint particles attached to said valve to project from its front end through the paint nozzle forward from the front end opening of the nozzle, and to which is applied a dc high voltage through a high-resistance element; and a change-over valve means for controlling supply of air to the air cylinder to operate the needle valve device such a conventional gun is constructed in such a manner that the rodlike valve of the needle valve device is formed into an integral body made of a metal and is adapted to function as a part of the conductor of the charging electrode, or it is formed into a tubular body made of a synthetic resin to have a center hole and the charging electrode is inserted into this center hole, or it is formed as a combined body consisting of a metal part which functions as a part of the electrode conductor and a synthetic resin part which functions as a valve member. Otherwise, the components of the needle valve device, i.e. the rodlike valve, the piston for driving the valve attached to the rear end of the valve, the air cylinder which accommodates the piston, and valve support members which are disposed in the cylinder to function as slide bearings and as retainers of the sealing for the valve are partly or entirely formed from metals.
In another example of the construction of the conventional air atomizing electrostatic coating gun of this type, the print nozzle which functions as a valve seat is fixed to the front end of the gun body, an air cylinder of the needle valve device is attached and fixed to the rear end of the gun body by inserting it into an axial hole formed in a rear portion of the gun body from the rear side thereof, and a rodlike valve having a generally cone-shaped front end portion extends forwardly through the paint chamber formed in a central portion of the gun body to engage its front end portion with a slanted inner wall of the paint nozzle.
The conventional air atomizing electrostatic coating gun also has a construction in which, in order to apply a dc high voltage through a current-limiting high-resistance element to the paint particle charging electrode in the form a straight wire passing through the paint nozzle and projecting forward from the front end opening of the nozzle, a comparatively large-sized rodlike resistor which has a high resistance is inserted into an axial hole formed through the gun body. Also the end portion of a high-tension cable which extends from a suitable direct current high-voltage generator disposed apart from the spray gun is introduced through a cable inlet hole into the gun body. The inner end of the cable conductor is connected to one end of the rodlike resistor, and the other end of the resistor is being connected to the charging electrode by a spring connector element.
However, if the valve in the needle valve device is partly or entirely formed of a metal, or if this part and/or other components of the needle valve device are formed of metal materials, it is, in fact, extremely difficult to completely insulate these metal components from the charging electrode. Thus, these metal components act as a part of the charging electrode, thereby increasing the capacitance thereof. There is therefore the risk of spark discharge occurring when the front end of the charging electrode is brought close to the human body, the object to be painted, a grounded metal member, etc., and, hence, the risk of injuring the human body or causing a fire by ignition of paint solvent. For complete insulation of the metal components, the size of the gun body is necessarily increased. If the whole body of the valve or the front end portion of the valve is formed of a synthetic resin, the sealing function of the front end member of the valve deteriorates so that the valve cannot effect accurate closing operation. Such deterieration is due to wearing of this member acting as a valve as the spray gun is used, due to its low hardness and wear resistance. The construction, in which the paint nozzle which acts as a valve seat is attached to the front end of the gun body and the needle valve device is disposed and fixed in a cavity formed in a rear portion of the gun body so that a generally cone-shaped front end portion of an elongated rodlike valve is engaged with the paint nozzle, is problematic in that it necessitates a laborious effort in dismantling the spray gun when the needle valve device needs to be checked or repaired. Also it is difficult for the valve to be brought into close contact with the valve seat due to an increased length of the valve. Also, there is a problem of the capacitance of the charging electrode being increased correspondently with an increase in the length of the charging electrode. In addition, the construction in which a direct current high-voltage generator disposed apart from the spray gun is connected to the spray gun by a high-tension cable to apply a dc high voltage to the charging electrode of the spray gun results in a spray gun which is difficult to handle smoothly and controllably. This is because an ordinary high-tension cable is heavy and has relatively low pliability of flexibility since it is generally formed with covering the conductor wires by several layers of insulating materials having good insulating properties, and fitting a helical metal tube around these insulating layers. In particular, there is a problem with of handling the gun smoothly when the spray gun is attached to an operating arm of a robot system so as to be automatically operated, and hence there is a possibility of the painting speed or the quality of the paint film being reduced.